A Traction (Friction) Curve Is Not a Flow Curve

• Published in: Lubricants 2022-09, Vol.10 (9), p.221
• Main Author: Bair, Scott
• Format: Article
• Language: English
• Subjects: Contact pressure; Elastohydrodynamic lubrication; Flow visualization; Friction; high pressure; High pressure research; Hydrocarbons; Hydrodynamics; Hydrofoil boats; Lubricants; Lubricants & lubrication; Lubrication; Lubrication and lubricants; Measurement; Methods; Mineral oils; Pressure dependence; Rheological properties; Rheology; Shear stress; Thick films; Traction; Viscoelasticity; Viscometers; Viscosity
• ISSN: 2075-4442; 2075-4442
• DOI: 10.3390/lubricants10090221

With the uncertainty regarding the global energy future, the ability to lubricate concentrated contacts with sufficiently thick liquid films while minimizing friction is of extreme importance. The assumptions of classical elastohydrodynamic lubrication have remained unchanged since the early days. It has not been possible to test many of these assumptions without the measurement of the viscosity at elastohydrodynamic lubrication (EHL) pressures, and viscometer measurements have been ignored. One of these assumptions has been the equivalence of a traction curve to a rheological flow curve for the lubricant. This notion should have been discarded forty years ago, simply because it required the pressure–viscosity behavior to be unlike the behavior observed in viscometers. At the heart of the problem is the fact that the pressure within the EHL contact is not homogeneous and the liquid properties are highly dependent on pressure, making the contact a very poor rheology laboratory. These past failures must be avoided in the future.